Degradation of dea treating solutions
Choy, Edward Takwah
1978-02-24

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The objectives of this research were (i) to develop an analytical method capable of measuring trace quantities of DEA degradation products and (ii) to study DEA degradation under carefully controlled experimental conditions.
The analytical method consisted essentially of removing water by air stripping, silylating the residue with N,O-bis(trimethylsilyl) acetamide (or "BSA"), and analyzing the silylated compounds by gas chromatography. An OV-17 stainless steel column and H₂ flame ionization detector were used. The effectiveness of this method was subsequently confirmed by its ability to analyze numerous DEA samples from laboratory experiments and industrial plants.
DEA degradation was first studied at atmospheric pressure using an all glass and teflon apparatus. When a 30% DEA solution was contacted with pure CO₂ for up to 23 days, no known DEA degradation products were detected. However, the DEA concentration was observed to decrease with time. This may be caused by the formation of heat-stable salts which are undetectable by gas chromatography.
DEA degradation experiments were then conducted at elevated pressures (up to 4238 kPa (600 psig)) and temperatures (165 to 185° C) using a stainless steel reactor. The results showed that DEA degradation proceeded rapidly in the presence of CO₂ and N,N-bis(2-hydroxyethyl) piperazine (HEP) was one of the major degradation products. Four other degradation products called T, X, Y and Z were also observed when the column temperature of the chromatograph was set to 142° C. The concentration of HEP was found to increase uniformly with time whereas concentrations of T and Y increased only slightly. The concentration versus time plots of compounds X and Z exhibited a maximum which suggests that they are degradation intermediates. The overall DEA loss was governed by a first order rate equation of the following form:
[See Thesis for Chemical Equation]
where A, E[sub a] and R are 2.03x10¹⁰ hour⁻¹, 2.17xl0⁴ cal. g-mole⁻¹, 1.99 cal g-mole⁻¹°K⁻¹ respectively.
A degradation mechanism is proposed which resembles that suggested by Polderman et al. (16):
[See Thesis for Chemical Equation]
where U denotes one, or more, unidentified compounds.
The theoretical concentration values of DEA and HEP based on the above mechanism fitted the experimental data fairly well. However, the predicted concentrations of HEOD did not exhibit a sharp maximum and therefore failed to match the behavior of X or Z.
The DEA samples obtained from the high pressure tests were also analyzed using a higher G.C. column temperature of 174° C. In this case, seven additional peaks were observed indicating other degradation products. One of the seven peaks could be caused by THEED, which was previously reported by Hakka et al. (17). It is therefore clear that the degradation of DEA is more complex than suggested by the above mechanism.
Special high pressure tests were conducted to discover the effects of acid gas composition, pressure, initial DEA concentration and metals on DEA degradation. It was found that CO₂ and temperature were the most important parameters affecting DEA degradation but purely thermal degradation was insignificant. CO₂ pressure appeared to be unimportant, between 2170 and 4238 kPa (300 to 600 psig), probably due to a large excess of CO₂. When the initial DEA concentration
was 10% (as opposed to 30%), the degradation of DEA proceeded more slowly than expected from the aforementioned rate expression. The reason for this behavior is presently unknown. When the DEA solution was saturated with H₂S at 1480 kPa (200 psig) and room temperature prior to contact with CO₂ at 4238 kPa (600 psig), it was observed that the degradation at 175° C decreased substantially. This was probably due to the fact that approximately half of the DEA combined with H9S and was hence protected against CO₂ attack. Initial results from preliminary paper chromatographic and mass spectroscopic studies are also reported. These studies provide guidance for future work.

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